always @(posedge a)
    begin
        repeat (1) @(posedge clk);
        fork： a_to_b
            begin
                @(posedge b)
                $display
                ("SUCCESS： b arrived in time\n", $time);
                disable a_to_b;
              end
            begin
                repeat (3) @(posedge clk);
                $display
                ("ERROR：b did not arrive in time\n", $time);
                disable a_to_b;
    end
         join
end
